Radiolabeled diethylstilbestrol (DES) was administered to one pregnant and three normal female rhesus monkeys. One normal female chimpanzee was also included in the study. Regardless of the mode of presentation (oral versus intravenous), the urine was the principal route of excretion for each species. The urine contained no non-polar radioactivity, and Sephadex LH-20 (MeOH/EtOH-50:50) resolved the radioactivity into five fractions (A, B, C, D, E). Fractions A,B, C, and D were hydrolyzable with beta-glucuronidase, and the principal aglycones were identified with GC/MS as cis-trans DES and dienestrol. The fecal excretory products were extracted with dimethoxy methane/methanol (50:50) and the radioactivity partitioned between benzene and H2O. The polar radioactivity was resolved by LH-20 (MeOH/EtOH-50:50) into chromatographic fractions similar to the urinary conjugates. These fecal conjugates were, however, less sensitive to beta-glucuronidase hydrolysis. The primary non-polar fecal radioactivity was chromatographically similar to DES (LH-20 and HPLC) in both species, and in the rhesus monkey the principal products identified were cis-trans DES and dienestrol.